Numerous criteria must be met by an alloy to be used in the fabrication of prosthetic dental appliances such as porcelain-veneered fixed bridgework and crowns. The alloy must be tissue tolerant, tarnish resistant, corrosion resistant and non-toxic. In addition, any oxide formed on the alloy surface should be adherent and not grow dramatically in thickness during the porcelain-fixing cycle. The oxides formed must also be compatible with the porcelain; otherwise, they may affect the thermal expansion of the interfacial porcelain. Still further, the oxides should not discolor the porcelain. The alloy must also have a coefficient of thermal expansion slightly higher than that of the porcelains currently available on the market thereby placing the porcelain under compression and minimizing the stresses formed at the interface.
The alloy also should be shape-stable with porcelain application, possess adequate strength for function, produce an acceptable fit and be solderable. Finally, it should possess a high modulus of elasticity, high-yield strength and hardness and be easily cast, ground and polished using techniques conventionally employed in dental laboratories. The criteria which govern the selection of a suitable alloy for use in the preparation of porcelain-veneered fixed bridgework and crowns are quite different from the criteria involved in selecting alloys for use in the fabrication of partial dentures which generally are not used in conjunction with porcelain.
These criteria, to a large extent, have heretofore been met by precious alloys containing gold, platinum, palladium, silver, indium, tin, gallium, zinc, and the like, and trace metals, such as those set forth in U.S. Pat. Nos. 1,283,264, 3,413,723, 3,667,936, 3,767,391, 3,819,366, 3,981,723 and 4,007,040 and the like.
With the ever increasing and fluctuating cost of precious metals and the superior physical properties and technological advantages offered by nickel-chromebase alloys, such alloys have become widely used as an alternative to precious alloys in dentistry. These alloys generally utilize tin, gallium and the like to impart specific physical characteristics. Typical of such alloys are those described in U.S. Pat. Nos. 2,089,587, 3,304,177, 3,464,817, 3,749,570 and 3,914,867.
Currently, there is growing concern about nickel being an allergen and beryllium being a toxic element. Although much data are still needed, there is an apparent need for a non-precious alloy which contains neither nickel nor beryllium and yet meets the above criteria. A number of cobalt-chromium base alloys with and without nickel and/or beryllium have heretofore been employed in dentistry for the fabrication of removable partials, crowns and bridgework. Typical of such alloys are those described in U.S. Pat. Nos. 3,756,809, 3,802,875, 3,802,934 and 3,837,838. However, their compositions and physical and thermal properties have limited their use for porcelain-veneered crown and bridgework. Cobalt-chromium based alloys having a variety of compositions and said to be useful for procelain-fused-to-metal restorations have been disclosed in U.S. Pat. Nos. 4,229,215, 4,253,869, 4,255,190 and 4,263,045.